Left Temporoparietal Neural Synchrony Tracks Cooperative Performance Across Adult Dyads

Cooperation requires interacting individuals to coordinate their actions toward shared goals, and interpersonal neural synchrony (INS) has been proposed as a neural correlate of this coordination. However, the relationship between INS and cooperative performance remains heterogeneous, and relatively little is known about how it varies across social relationships. The present study examined INS and behavioral performance during a naturalistic cooperative tabletop Jenga task in close-friend, romantic-partner, and mother--child dyads (N = 284 participants). Brain activity was simultaneously recorded using functional near-infrared spectroscopy (fNIRS) hyperscanning, and INS was quantified using wavelet transform coherence across the bilateral inferior frontal gyrus (IFG) and temporoparietal junction (TPJ). Cooperative performance was assessed using tower collapse and the number of valid turns completed during the game. True interacting dyads exhibited significantly greater neural synchrony than surrogate dyads across all examined regional pairings (d range = 0.35-0.55), indicating robust interaction-related inter-brain synchrony. Behavioral performance was not associated with INS at the whole-network level. Region-specific analyses, however, showed that tower collapse was associated with reduced synchrony in the left TPJ-left TPJ (p Bonf = 0.009) and left TPJ-right TPJ (p Bonf = 0.017) pairings, whereas the number of valid turns was unrelated to INS. Follow-up analyses further indicated that the association between tower collapse and left TPJ--left TPJ synchrony was present in close-friend (p Bonf = 0.020) and romantic-partner (p Bonf = 0.026) dyads but not in mother-child dyads. These findings suggest that the behavioral relevance of interpersonal neural synchrony is not uniform, but depends on the neural systems involved, the specific index of cooperative performance, and the relational and developmental context in which interaction unfolds.